In recent years, metal oxides having semiconductor characteristics (hereinafter, referred to as oxide semiconductors) have attracted attention. Oxide semiconductors may be applied to transistors (see Patent Documents 1 and 2), for example.
There are many types of transistors. For example, transistors may be classified as a bottom gate-type structure and a top gate-type structure according to the positional relationship among a substrate, a gate, and a channel formation region. A transistor structure having a gate placed between a channel formation region and a substrate is called a bottom gate-type structure. A transistor structure having a channel formation region placed between a gate and a substrate is called a top gate-type structure.
In addition, transistors may be classified as a bottom contact type and a top contact type according to connection portions of a source and a drain with a semiconductor layer in which a channel is formed. A transistor with a structure where the connection portions of a source and a drain with a semiconductor layer in which a channel is formed is placed on a substrate side is called a bottom contact type. A transistor with a structure where the connection portions of a source and a drain with a semiconductor layer in which a channel is formed is placed on a side opposite to a substrate (that is, a counter substrate side in a liquid crystal display device) is called a top contact type.
Types of transistors can be classified as a BGBC (bottom gate bottom contact) structure, a BGTC (bottom gate top contact) structure, a TGTC (top gate top contact) structure, and a TGBC (top gate bottom contact) structure.